What is in this article?:

High-frequency measurement systems based on vector network analyzers (VNAs) are available for analyzing the performance of multiport active and passive components well through millimeter-wave frequencies. But their high resolution and broad frequency ranges mean very little if these systems are not accurate, and that requires proper calibration practices. Fortunately, automatic calibration methods can help maintain the accuracy of VNA-based systems without the cost of excessive test-system downtime or lost production measurement throughput.

Four-port RF/microwave VNAs are available from numerous test-equipment suppliers in various configurations, delivering superb performance in terms of scattering (S) parameters, amplitude, and phase for active devices; active and passive components; and even integrated subassemblies. They can also provide high accuracy, but that accuracy relies on proper calibration of the test system. A typical VNA-based automatic-test-equipment (ATE) system includes test cables and switches that add some loss, group delay, and dispersion that must be removed from the final measurement results displayed for a multiport device under test (DUT). A proper calibration should remove the electrical contributions of these additional components and modules that are necessary parts of the test signal path.

To maintain test-system accuracy, a calibration should be performed each day, with the test system calibrated at the DUT ports. Unfortunately, manual calibration procedures are very time-consuming and prone to errors. In addition, the time required for a manual calibration means less time for the test system to evaluate DUTs that may be running through final test procedures prior to shipment to a customer. The time required for calibration takes away from the system throughput and must be minimized to reduce the cost per tested unit.

Of course, without a calibration, or with a poorly performed calibration, a test system will yield poor measurement accuracy, and this can be devastating to a company in many ways. When a VNA-based test system is not properly calibrated, and the accuracy is low, the measurement uncertainty can result in DUTs that do not meet specification being approved for final shipment, as well as some DUTs that actually meet their minimal specification limits being failed because of the poor measurement accuracy. Both cases can be costly to a manufacturer. Shipping parts that do not meet required performance specifications will result in failures for a customer’s end products that incorporate those DUTs, and the resulting difficulties in customer relations because of those failures. Also, discarding parts that might have exceeded those minimum performance requirements results in increased material costs.

Perhaps the most popular VNA calibration approach is the short-open-load-through (SOLT) method, which is typically used for a two-port instrument calibration and relies on measurements of well-defined passive circuit standards to establish the accuracy of the test system across its operating frequency range. As part of the SOLT calibration process, the measured characteristics (including amplitude and phase response and S-parameters) of each reference standard are compared to the known values of those parameters for the standard. As noted, however, it can be extremely time-consuming to perform, with the need to switch among the four standards, and it can be prone to mistakes that lead to measurement/calibration errors (see table).

Commercial calibration kits with the physical standards are available from a number of suppliers, or else standards can be constructed based on known electrical/mechanical parameters—such as the electrical length of a transmission line at a given center frequency. Once the reference standards are measured, an RF/microwave VNA compares the measured values with the known values for each standard, and computes a correction factor for each measured frequency point based on well-defined calibration equations. These correction factors are then applied during subsequent VNA measurements.

The measurement or reference plane is defined by the physical location where the calibration standards have been connected to the system, and this is also the point at which the correction factors are applied to begin a measurement. If coaxial cables were used during the calibration, with the cables connecting the reference standard to the VNA, then the reference plane is at the end of the coaxial cables.